This issue is caused due to credential delegation. By default, when you authenticate to remote server (via Enter-PSSession), these credentials can be used only on that server. Server is not allowed to authenticate you anywhere else, while Get-CertificationAuthority cmdlet does some remote requests to LDAP and CA server. You have either, to use CredSSP or enable this server trusted for delegation.
As aside note: the server part of the module is not designed to run in remoting session, instead each command in server cmdlets has parameters to work with remote CAs.
As aside note: the server part of the module is not designed to run in remoting session, instead each command in server cmdlets has parameters to work with remote CAs.